The invention relates to receiving signals from a transmission medium, and more particularly to receiving acoustical signals from a borehole drilling apparatus.
Transmission of signals in a medium such as a drill pipe by means of acoustical energy has been found to be a convenient way of monitoring parameters in a borehole. Acoustical telemetry systems have been recently disclosed by Elbert N. Shawhan in his U.S. patent applications entitled "Telemetering Systems for Boreholes" bearing Ser. No. 390,833 and filed Aug. 23, 1973; and "Two-Way Acoustical Telemetering System" bearing Ser. No. 416,467, filed Nov. 16, 1973. These and other systems in which signals are sent through a solid medium by means of acoustical energy require appropriately located means for receiving the signals from the medium.
In the borehole systems it is preferable that the signal be extracted from the transmission medium at a point in the drilling apparatus or rig that will not interfere with drilling operations. It has been proposed that the signal be picked off at the upper end of the drill pipe. However, this would present several serious problems. First, in a rotary drilling system some means of connecting a conductor line to the rotating pick-up apparatus must be found if telemetry is desired while drilling is occurring. Second, if such a connector is not found, drilling must be stopped to perform telemetry, thereby wasting costly drilling time. Third, in either the above two situations the pick-up must be periodically attached and removed, which consumes further drilling time. In drilling operations conducted with a turbine-driven drill, in which the drill pipe does not rotate, the drilling operation is hampered primarily by attachment and removal of the pick-up. Attachment of the pick-up to the drill pipe is therefore unsatisfactory, and another location for the pick-up is desirable.
Among the possible locations for the pick-up in a drilling apparatus, some of the more convenient are generally pipes or other cylindrical members. A pick-up for use on such cylindrical members should provide good acoustical coupling. Welded coupling devices have been proposed, but they do not provide the flexibility of placement that is desirable. Non-welded coupling devices that have been proposed for such members generally include a block having an arcuate face to mate with the cylindrical surface and a flexible strap which wraps around both the block and the cylindrical member to secure the two together. A pick-up of this design, however, provides in some cases a poor medium for passage of precise acoustical signals due to the position of the strap. Unless the block precisely mates with the cylindrical member and its lateral edge is perfectly tangent to the cylindrical surface, there is a gap over which the strap must pass between the block and the cylindrical surface. The strap is thus not in intimate contact with the solid surface in this area and is free to vibrate. Vibration introduces undesirable resonance into the pick-up apparatus, which distorts the signal.
The desired mode of contact is not generally possible due to variations in size and shape of cylindrical members to which the block may be attached. For example, the block may fit a new piece of drill pipe within limits of tolerance. However, as the pipe ages, it may wear down to a slightly smaller size and become bent and misshapen, thereby precluding the desired mode of contact. The wrap-around strap design is therefore unsatisfactory in such situations, and a pickup whose reception characteristics are not altered by variations in drill pipes, or either cylindrical members, is desirable.